1. Field of the Invention (Technical Field)
The present invention relates to optical inspection of microelectronic devices, in particular measurement of line profile asymmetry using scatterometry.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-à-vis the present invention. Discussion of such publications herein is given for more a complete understanding and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
The fabrication of a microelectronic device is a complicated procedure that uses a variety of equipment for the different process steps involved. First, the lithography process transfers the image being made into a light sensitive material known as photoresist. This image in photoresist, in turn, acts as a mask for the next patterning process known as etching. Etching is the process by which the resist image is transferred into a suitable material such as poly-silicon. Then the etched material is over-filled with some insulating materials, planarized if necessary, and the whole process begins again.
Throughout the entire process the devices being made should be symmetric in nature from step to step, i.e., a correctly manufactured transistor gate will have equal left and right sidewalls as well as other features such as, but not limited to, equal left and right corner rounding. If errors occur during the processing, this desired symmetry may be compromised, and as a result the device integrity or functionality may also be compromised. If the asymmetry is quite severe the device may not function at all.
The present invention relates to performing symmetry/asymmetry measurements via scatterometry. Scatterometry is an optical inspection technique well suited for the measurement of symmetry or asymmetry on microelectronic devices. By analyzing the light scattered from an array of microelectronic features, measurements of the line profile can be made. In particular, a scatterometer that measures at complementary angles, i.e., +45 degrees from a position perpendicular to the surface and −45 degrees, is ideally suited for symmetry/asymmetry measurements because the reflectance properties of the line profile can vary at these angles, although complementary angles are not necessarily needed to detect asymmetry. To enhance the sensitivity of this effect the array of features should be placed in a particular orientation, known throughout the specification and claims as a general conical configuration, namely one in which the wave vector of the illuminating beam does not remain parallel to the array's plane of symmetry.
Prior art techniques typically employ “classic” scattering. These are measurements geared towards the measurement of surface roughness, defects, pitting, etc. However, the present invention is based on the physics of diffraction, with the measurements in the invention always occurring with respect to periodic features (such as line/space gratings).
Prior work in scatterometry used the technique for the measurement of line profiles in resist and etched materials. C. J. Raymond, et al., “Resist and etched line profile characterization using scatterometry,” Integrated Circuit Metrology, Inspection and Process Control XI, Proc. SPIE 3050 (1997). Embodiments of the present invention provide techniques for the measurement of asymmetric line profiles (e.g., unequal sidewall angles).